A disturbance of prostaglandin metabolism is currently believed to be one of the major mechanisms in the pathogenesis of preeclampsia. Most previous studies of this theory, however, have included only women who had already developed clinically overt disease. In order to investigate whether prostaglandin abnormalities precede the onset of preeclampsia, we have designed a nested case-control study using stored urine specimens prospectively collected as early as thirteen weeks gestation from women participating in the NICHD Trial of Calcium to Prevent Preeclampsia (CPEP). In this study, we will compare the urinary excretion of prostaglandin and thromboxane in preeclamptic women before the diagnosis of preeclampsia to that in normal pregnant women at comparable gestational ages. Because half of the women in the CPEP trial will be taking two grams of oral calcium daily, we will also be able to examine the effect of oral calcium supplementation on the excretion of these prostaglandin metabolites.